This invention relates to piezopolymer actuators and specifically to application of such actuators in tactile arrays.
In the frequency region between 250 and 300 Hz, the fingertips are very sensitive to vibratory motion. It is sufficient to have an amplitude of vibration in the order of 10 microns to recognize patterns. Tactile arrays composed of closely packed columnar actuators with center-to-center spacing of 1-2 mm are very desirable for the design of a Braille-type reader or, in fact, for a general computer display device for the blind. Dense arrays can also be used to convert sound patterns to tactile patterns and thus substitute for hearing for the deaf.
Commercially available OPTACON tactile stimulator for the blind, manufactured by Telesensory Systems, Inc., in Mountain View, Calif., uses a 144 actuator array. Individual actuators are made from a piezoelectric ceramic, lead zirconite titanate (PZT). While PZT is an effective material for the transformation of electrical energy to mechanical energy, PZT is nonetheless heavy and fragile and therefore not well suited for a portable instrument in a rough environment. Furthermore, ceramic actuators, because of their size, cannot easily be formed into large, dense arrays.
Each piezoelectric actuator is assembled by hand with epoxy, and separate contact wires are required for each element. The cost of assembling the tactile array is therefore high since the manufacture does not lend itself well to mass production. Significant increase of the density and the number of elements, beyond the recently available one hundred and forty-four, is not practical due to limitations of the above state-of-the-art.
Piezoelectric polymers, such as polyvinylidene difluoride (PVdF) film, are attractive alternatives to ceramic piezoelectrics for an actuator design. Pennwalt Corp., among others, manufactures PVdF as a metallized pliable film, available in 9-52 microns thickness, under the trade name of Kynar. The properties of the PVdF piezoelectric polymer are very different from ceramic piezoelectrics. PVdF is less dense and much more pliable than ceramic piezoelectric materials. Its piezoelectric charge coefficient is about one tenth of that for lead zirconate titanate (PZT). However, the alternating field strength which can be applied to PVdF without depolarization is one hundred times greater than that which can be applied to PZT, resulting in ten times greater maximum linear strain. (Maximum linear strain is the ratio of elongation to the length of the piezoelectric element corresponding to the maximum operating electric field.)
The actuators discussed here use PVdF in the transverse mode: The application of a voltage across two metallized electrodes deposited on the faces of the film causes the material to expand or contract along the axis corresponding to the direction of stretch during film fabrication.
The theoretical potential of PVdF as a tactile array element has been recognized by Prof. J. G. Linvill of Stanford University, whose earlier research led to the development of the OPTACON. Because of its pliability, PVdF is ordinarily used as a tension member rather than as a compression member. Linvill initially hoped that PVdF wound into a free-standing cylindrical tube could be used in a transverse mode as a compressive strut. Such tubes can be closely packed into a dense multi-element two-dimensional array.
The results of the investigation of freestanding cylindrical struts has been reported in an article by D. H. Dameron and J. G. Linvill entitled "Cylindrical PVF.sub.2 Electromechanical Transducers" in Sensors and Actuators, Vol. 2 (1981/82) pp. 73-84. Construction of such a tubular strut is shown in FIG. 1, which is substantially a replica of FIG. 3 in the above article.
Dameron and Linvill discuss a number of serious problems which they encountered which eventually resulted in abandoning any attempt to manufacture the device in FIG. 1. The metallized electrodes deposited on the film of PVdF consist of a layer of nickel-chromium approximately 0.06 microns thick. Winding the flat-stock PVdF film into a small radius cylindrical strut produces creases in the film which cause small cracks in the electrodes. These cracks electrically isolate islands of metallization on the film, preventing some portions of the electrode surface from being energized. The damaged electrode regions quickly fail due to the higher non-uniform current densities and weakened bonding caused by the cracks. This problem is particularly severe with 8-9 micron PVdF film. It caused Dameron and Linvill to abandon this thin material even for laboratory devices, in favor of 30 micron PVdF film.
The maximum electrical field which can be repeatedly applied to PVdF film is 30 volts per micron. For maximum linear strain, an actuator wound from 8 micron material requires a peak voltage of .+-.240 volts. The thicker film of 30 microns which Dameron and Linvill used requires a peak voltage of .+-.900 volts. In anything but a laboratory demonstration, it is very difficult and expensive to design hundreds of drivers for a tactile array with the capability of a 1800 volt swing. On the other hand, a 480 volt swing can be handled with presently available solid-state power drivers.
A general problem in high linear strain mode of operation, regardless of PVdF film thickness, is that the metallized electrodes clearly do not stretch as the underlying PVdF substrate shrinks and elongates. Consequently, the electrodes are likely to break up into islands, and electrical conductivity between these islands becomes haphazard and unreliable.
The actuator in FIG. 1 requires two connections per actuator. These connections are accomplished by bonding pads provided on the actuator to printed circuit board traces, using conductive epoxy. A major problem is loss of electrical connection due to high current density at the bond points. This problem is further aggravated by the difference in thermal expansion between the PVdF, the electrode material, and the conductive epoxy causing microcracks to form in the electrode metallization and leading to contact failure. Electrical connection to an actuator in FIG. 1 is a difficult manual operation largely negating any assembly advantage over PZT actuators.
Linvill summarized the state of the art of piezopolymer cylindrical struts (J. G. Linvill "Piezoelectric Polymer Transducer Arrays", Proceedings of the Sixth IEEE International Symposium on Applications of Ferroelectrics. (1986) pp. 506-510) by saying:
"The obvious limitation of the cylindrical strut as an element in a piezoelectric array is the complication in its construction. These elements made in conjunction with the work reported in Reference 4 (Dameron & Linvill) were constructed singly and by hand. Arrays of larger numbers of elements using these methods never appeared feasible."
Thus, there is a need for a reliable and durable actuator of a long cylindrical form which can be densely packed into an array and which can be mass produced.